Reactive gas transport through a channel differs in the molecular flow (collisionless) regime from the flow in a fluid (collisional) regime. Chemical systems composed of gas and surface species and elementary reactions on the surface are simulated in the collisionless transport regime by CKnudsen, a new Chemkin code. Angular distributions for incident flux from all sources for each gas species are assembled at each point of the surface which encloses the volume. The system of simultaneous reaction rate equations is solved deterministically at each surface point. The reaction rates at each surface point together with the input angular flux distribution for each gas determine the angular distribution of reemitted flux for each gas. The use of the same Chemkin reaction formalism and subroutine libraries used by fluid codes facilitates multi-scale simulation and the validation of proposed reaction mechanisms in different regimes. As an example, Arora and Pollard's W CVD mechanism with 16 elementary surface reactions@footnote 1@ is converted into Chemkin format and evaluated at the equipment scale and then at the feature scale in submicron trenches and compared to experimental results. @FootnoteText@ @footnote 1@ R. Arora and R. Pollard, J. Electrochem. Soc. vol. 138, 1523-1537 (1991).